The development of fuels from renewable agricultural sources is currently and will likely continue to be important in meeting future energy demands and reducing the production of greenhouse gas emissions from fossil carbon sources. Current “biofuels” under development include “biodiesel” derived from vegetable oil and ethanol fermented from sucrose obtained from plants such as corn and sugar cane.
A potentially plentiful source of biomass for the creation of biofuels is plant fiber found in agricultural residues such as corn stover and bagasse or crops grown specifically for fuel production including hardwood and softwood trees, among other sources. More specifically, the cellulose polysaccharides found in the plant fiber can be broken down into simple sugars and subsequently fermented to produce ethanol.
In order to promote the effect of cellulase enzymes in “saccharification” of cellulose in order to place the cellulose polysaccharides in condition for being broken down into the simple sugars for fermentation, it is necessary to remove the cellulose from its protective lignin. A number of techniques exist for mechanically removing the lignin and exposing the cellulose, including for example steam explosion techniques. In addition, the Kraft or sulfite pulping process removes lignin from the cellulose fibers by treatment with sodium hydroxide, sodium sulfide, or salts of sulfuric acid as a predicate to papermaking. In conventional papermaking, after the lignin is removed from the cellulose, the lignin is burned in boilers to recover the inorganic pulping chemicals used to remove the lignin and provide energy to be utilized for operating the pulping process.
In an alternative process to the Kraft or sulfite processes, ethanol may be used in an aqueous solvent extraction process to dissolve the lignin from the plant fiber, thereby removing it from the cellulose. This solvent extraction of lignin may be preceded by a hydrolytic cleavage of the lignin into smaller molecular weight fragments to improve its solubility. In this “organosolv” process, the extracted lignin, uncontaminated by the harsh pulping chemicals utilized in the Kraft or sulfite processes, maybe broken down into various products, such as the biofuel ethanol, which can be sold for a variety of commercial purposes, or used as a source of ethanol for the organosolv process, as illustrated in FIG. 4. Such organosolv processes include the Alcell® process described in U.S. Pat. No. 4,764,596, incorporated by reference herein, and improvements of which are commercially available from Lignol Innovations Limited of Canada.
However, when utilizing the organosolv process, after removal of the cellulose from the lignin, the lignin and the aqueous ethanol solvent are separated from one another in order to be able to recycle the ethanol as the solvent for the organosolv process, and to enable the remaining components to be used to form the other products via various separation processes, as illustrated in FIG. 4. It is necessary to employ a number of other devices and apparatuses to perform these separation processes, many of which require a significant amount of energy in order to perform the separation. In particular, because the solvent used to separate the lignin in the organosolv process is ethanol, which is very soluble in water, a highly energy-intensive distillation process is required to separate the ethanol to recycle the ethanol for use as the solvent in the organsolv process, or to use the ethanol as a biofuel.
As a result, it is desirable to develop a process for separating lignin from cellulose that is applicable to various sources of biomass material, which is less energy-intensive than prior lignin separation processes and that produces a high energy density biofuel.